Effect of dynamic plastic deformation on microstructure and annealing behaviour of modified 9Cr−1Mo steel

The effect of dynamic plastic deformation on the microstructure of a modified 9Cr?1Mo steel has been investigated in comparison with the effect of quasi-static compression. It is found that the boundary spacing after dynamic plastic deformation is smaller and the hardness is higher than those after quasi-static compression. The microstructure after dynamic plastic deformation is however less stable than the microstructure after quasi-static compression. Annealing at 675 and 700°C leads to structural coarsening and recrystallisation in each sample, but with recrystallisation occurring faster in the sample annealed after dynamic plastic deformation. The lower thermal stability of the microstructure produced by dynamic plastic deformation is attributed to a higher driving force for recrystallisation in the dynamically deformed material.     

Effect of tempering temperature on monotonic and cyclic properties of a high–strength low–alloy chain steel

Abstract

The influence of tempering temperature in the range 200–600°C on the monotonic and cyclic response of a high-strength, low–alloy steel used for haulage chains has been investigated. Static properties are little affected by tempering up to 400°C, with a small loss of strength arising from carbide coarsening. At higher tempering temperatures, dislocation annihilation is substantial and the strength falls markedly. On undergoing high–strain fatigue, the alloy exhibits cyclic softening at all tempering temperatures, but with a maximum of about 25% after a 400°C temper. The microstructural changes producing this are difficult to detect directly, but it is thought that dislocation unpinning has an important role. For tempering temperatures in excess of 400°C, the reduced softening can clearly be equated with a dislocation redistribution and the formation of cell substructure within the ferrite grains. Estimations of the magnitude of the various components of strength are made.

MST/199     

Effect of modified austemper on tensile properties of 0·52%C steel

Abstract

The effect of a modified austemper on the tensile properties of 0·52%C steel has been studiedfor the purpose of developing the mechanical properties of upper bainitic steel. The modified austempering treatment involved intercritical annealing at 1018 K in the two phase region offerrite (α) and austenite (γ) followed by austempering at 673 K and subsequent water cooling. The results have been compared with those obtained from conventionally austempered steel, and quenched and tempered steel with a similar ultimate tensile stress. The modified austempered steel consisted of a mixed structure of upper bainite and 10 vol.-% ferrite in which ferrite appeared as layers along the rolling direction. The modified austempering treatment wasfound to significantly increase the product of ultimate tensile stress and total elongation, and also the notch tensile stress at 193 K. Conventional austenitising at 1173 K followed by subcritical annealing at 998 K in the two phase region of ex and y, and then austempering at 673 K and subsequent water cooling produced the same mixed structure of upper bainite and 10 vol.-% ferrite. However, this treatment yielded inferior mechanical properties to those obtained with the modified austempering treatment, independent of the test temperature. The results are described and discussed.

MST/3102     

Effect of tempering temperatures on microstructures and properties of 0.28C–0.22Ti wear-resistant steel

The microstructures and properties of a 0.28C–0.22Ti low-alloy wear-resistant steel at different temperatures from 200 to 600°C was experimentally studied. It is shown that the wear resistance of the steel is not monotone changing with its hardness and strength. With the increase of the tempering temperature, the tensile strength and the hardness of the steels were gradually declined; however, the wear resistance was first decreased and then increased. The TiC particles can be divided into two classes: the small TiC particles (about 0.3–0.4?µm in diameter) and the coarse TiC particles (1–5?µm in diameter). The small TiC particles can improve the yield strength of the steels, and the coarse TiC particles can improve the wear resistance of the tested steels.     

Crack growth behaviour of P92 steel under creep and creep–fatigue conditions

Abstract

High temperature creep and creep–fatigue crack growth tests were carried out on standard compact specimens machined from ASME P92 steel pipe. The effects of various loading conditions on crack growth behaviours were investigated. Crack initiation time was found to decrease with the increasing initial stress intensity factor under creep condition and further to decrease by the introduction of fatigue condition. For creep test, the crack growth rate can be well characterised by the facture mechanics parameter C*. For creep–fatigue test, the crack growth behaviour is dominated by the cycle dependent fatigue process when the hold time is shorter, but it becomes dominated by the time dependent creep process when the hold time becomes longer.     

Effect of test temperature on fracture toughness of modified 9Cr–1Mo steel

Abstract

The quasi-static fracture behaviour (J–R curves) of modified 9Cr–1Mo (P91) steel was studied. The J–R curves were established at 298, 653, 823 and 893 K, and fracture toughness J_0·2 at 0·2 mm of crack extension was determined. The value of ～J_0·2 at 653 K was lower compared to that at 298 K followed by increases in J_0·2 values at 823 and 893 K. The decrease in J_0·2 at 653 K can be attributed to the influence of dynamic strain aging. At 893 K, a significantly higher (more than 200%) J_0·2 was observed, since plastic deformation of the net section, rather than crack growth, occurred in this condition.     

Effect of weld thermal cycle on behaviour of Ti–Nb carbonitrides in HSLA steel

Abstract

The effect of weld thermal cycle on the stability of carbonitrides has been investigated for an HSLA steel containing Ti and Nb. The precipitates at various distances from the fusion line of a bead on plate weld were characterised using energy dispersive X-ray analysis and electron energy loss spectroscopy. The results showed that starlike precipitates originally present in the strand cast condition had survived the controlled rolling treatment, but weld thermal cycling caused dissolution of the ‘arms’ to a varying extent depending on the distance from the fusion line. It was also shown that larger ( ≥ 50 nm) Ti, Nb particles became richer in Ti on approaching the fusion line and displayed a compositional gradient from core to skin, the skin in all examples showing a greater enrichment in Ti than the core. Equilibrium thermodynamics can be used to make reasonable predictions of particle composition following processing; however, for low heat input welding it is necessary to take diffusional kinetics of individual elements into account if compositional changes are to be understood.

MST/1022     

Effect of W–Mo balance on long-term creep life of 9Cr steel

The effect of tungsten–molybdenum (W–Mo) balance on creep life has been investigated for five heats of martensitic 9Cr steel with 1.5 % Mo equivalent (= 1/2W + Mo) at 600, 650 and 700°C. The combination of W and Mo concentrations in the present steel is 3W–0Mo, 2.8W–0.1Mo, 2.4W–0.3Mo, 1.8W–0.6Mo and 0W–1.5Mo. The time to rupture t_r exhibits a monotonous increase with increasing the W–Mo balance parameter 1/2W/(1/2W + Mo), namely, with increasing W concentration and concomitantly with decreasing Mo. The increase in t_r with increasing 1/2W/(1/2W + Mo) becomes less significant at long times. The precipitation of Fe₂(W,Mo) Laves phase takes place preferentially at prior austenite grain boundaries during creep, which enhances the grain boundary (GB) precipitation hardening. The amount of Laves phase increases with increasing 1/2W/(1/2W + Mo). The coarsening of Laves phase takes place at long times during creep, which reduces the GB precipitation hardening.     

Investigation on influence of dynamic strain ageing on fatigue crack growth behaviour of modified 9Cr–1Mo steel

Fatigue crack growth behaviour of modified 9Cr–1Mo steel is examined in the temperature range 300–823 K. An improvement in fatigue crack growth resistance is observed in the dynamic strain ageing regime. The activation energy for the process leading to this is estimated from the temperature-dependence of crack tip strain rate as 55–80 kJ/mole. This indicates that dynamic strain ageing due to interaction of dislocations with interstitial solute elements is responsible for the improved fatigue resistance in this range.     

Effect of thermal treatment on the evolution of delta ferrite in 11Cr–3Co–2.3W steel

Effect of thermal treatment on the evolution of delta ferrite was studied in 11Cr–3Co–2.3W steel. Ferrite-forming alloy elements promoted the formation of delta ferrite during thermo-mechanical processing. The results of JMatPro and chromium equivalent calculations indicated that 11Cr–3Co–2.3W steel partially contained delta ferrite. Cr and W were segregated in/around delta ferrite. Delta ferrite had a bamboo-type morphology and it contained MX-type carbonitrides inside and the Laves phase at the interface between delta ferrite and martensite. On annealing, delta ferrite dissolved especially during the early stage of annealing. After ageing at 600°C for 500?h, some Laves phases were observed in delta ferrite which is regarded as a favourable nucleation site. In the meantime, its precipitation resulted in the reduction of dissolved W and Mo in the matrix.     

Effects of friction stir processing on wear properties of WC–12%Co sprayed on 52100 steel

A WC–12%Co coating was thermally sprayed on a 52100 steel substrate and subsequently friction stir processing (FSP) was performed on this layer. The wear resistance and hardness was compared before and after FSP. Optical and SEM revealed that FSP intermixes the sprayed layer with the substrate, reduces porosity, and enhances both hardness and wear performances. 3D profilometry mapping was conducted to evaluate the wear track depth and its morphology. Refined grain structures and a martensitic structure with retained austenite are promoted by the FSP treatment. This leads to formation of new intermetallic and carbides which were detected by X-ray diffraction, thus accounting for the increased hardness and improved wear resistance.     

Effect of long term elevated temperature service on the properties of type P–22 steel

Abstract

Extensive service at 540°C led to changes in the composition, morphology, size, and distribution of the original carbides that were present in the pre-service steel. This service did not seem to introduce weakening voids along grain boundaries. At low creep stress levels the stress versus Larson–Miller parameter (LMP) curve of the postservice steel indicates better performance than the lower bound of the pre-service steel. The opposite performance was found at high creep stress levels. The service led to only a slight reduction in fracture toughness of the steel. Exposure to post-service accelerated creep tests led to further gradual reduction in fracture toughness. The reductions in fracture toughness were proportional to the creep strain and duration. The formation of spherical carbides during the accelerated creep tests promoted void formation during the fracture process and thus contributed to the observed additional reduction in fracture toughness.     

Effect of microstructure on the fatigue crack growth behaviour of a near-α Ti alloy

Fatigue crack growth behaviour of Ti–6Al–2Zr–1.5Mo–1.5V (VT-20 a near-α Ti alloy) was studied in lamellar, bimodal and acicular microstructural conditions. Fatigue crack growth tests at both increasing and decreasing stress intensity factor range values were performed at ambient temperature and a loading ratio of 0.3 using compact tension samples. Lamellar and acicular microstructures showed lower fatigue crack growth rates as compared to the bimodal microstructure due to the tortuous nature of cracks in the former and the cleavage of primary α in the latter. The threshold stress intensity factor range was highest for acicular microstructure.     

Effect of silicon on recrystallisation of V–Ti microalloyed steel

Abstract

The dynamic recrystallisation (DRX) and static recrystallisation (SRX) behaviours of three V–Ti microalloyed steels were studied by the analysis of the true stress–strain curves and the stress relaxation curves under different deformation conditions. The results of DRX showed that deformation activation energy Q_def, peak stress and peak strain increased, as a result of the solute strengthening and dragging effect due to Si. The results of SRX showed that Si increased the SRX activation energy Q_SRX. The solute retardation parameter for static recrystallisation of Si was calculated. Based on the SRX results, to quantify the drag effect of Si and V, a new model was proposed to describe the time for 50% recrystallisation (t_0·5), which was tested and verified by previously published data on similar steels. Precipitation during recrystallisation could lead to a lower value of the Avrami exponent.     

Effect of grain size on tensile behaviour of a submicron grained Al–3 wt-%Mg alloy produced by severe deformation

Abstract

Equal channel angular extrusion has been used to deform an Al–3 wt-%Mg alloy to an effective strain of 10, resulting in a 0.2 µm grain size. In the as deformed condition the yield strength was increased to ～500 MPa. During annealing the grain structure coarsened uniformly and the yield stress was found to follow the Hall–Petch relationship, even in the submicron range. There was an abrupt transition in elongation at a grain size of ～0.5 µm. Samples with smaller grain sizes showed no uniform elongation and limited ductility. For slightly greater grain sizes there was only a relatively small reduction in elongation, compared to a coarse grained material, while the yield stress was still increased by a factor of over four. Reducing the grain size to the submicron range led to far higher Lüders strains than are normally observed in Al–Mg alloys.     

Effect of heat treatment on creep and fracture behaviour of 1·25Cr–O·5Mo steel

Abstract

The effect of tempering treatments on the microstructure and creep behaviour of multipass 1·25Cr–0·5Mo steel weldments has been evaluated. While tempering invariably reduced the hardness, significant changes in microstructure were only found after heat treatment at 750°C. In this case ferrite bands developed adjacent to the fusion boundary. Tempering increased creep deformation and reduced failure lifetime for base metal specimens. For crossweld testpieces, the susceptibility to low ductility failures in the heat affected zone was found to be linked to the development of creep cavities and cracks. Thus, brittle failure modes were a function of stress, temperature, microstructure, post weld heat treatment, and to a lesser extent, specimen geometry.

MST/3069     

Effect of holding time on vacuum brazing for a stainless steel plate–fin structure

This paper presents a vacuum brazing of 304 stainless steel plate–fin structures with nickel-based BNi-2 filler metal. The effect of brazing holding time on tensile strength and microstructure has been investigated, aiming to obtain the optimal brazing holding time. The microstructure in brazing joint consists of diffusion-affected zone (DAZ), interface reaction zone (IRZ), isothermally solidified zone (ISZ) and athermally solidified zone (ASZ). The structure in the fillet is composed of solid solution, nickel silicon, nickel boron compound and a mixture with nickel silicon and nickel boron. The tensile strength increases along with the increase of holding time, but decreases when the holding time is over 25 min. A maximum tensile strength of 65.1 MPa is obtained with 25 min holding time. Too short holding time will make boron diffuse insufficiently and generate a great deal of brittle boride components, and too long holding time will make the base metal dissolve into the filler metal excessively and creates more corrosion voids.     

Effect of laser heating on substructure of 0· 4%C steel

Abstract

It is expected that surface treatment by CO₂ laser will be useful for industrial applications in which improved wear resistance is required on selected areas of already accurately machined parts. Precision in the hardening of such surfaces using laser processes is an additional advantage. Moreover, it is possible to obtain these desirable properties for low cost construction steels. The microstructural changes within the laser heated zones of commercial grade medium carbon C45 steel were determined on thin foils from the treated regions using transmission electron microscopy. Components of the substructure are described and the influence of rapid laser heating on austenite formation and its transformation into martensite are discussed.

MST/563     

Equal-Channel Multi-Angle Pressing Effect on the Properties of NbTi Alloy

An improvement of NbTi alloy functional properties by equal-channel multi-angle pressing (ECMAP) combined with hydrostatic extrusion, drawing and thermal treatment is revealed. The ECMAP method allows to increase the billet accumulated deformation with preserving its initial dimensions. The formed highly dispersed and homogeneous nanocrystalline structure with a more uniform distribution of α-phase precipitations as a result of ECMAP treatment improves the functional properties of the alloy. In the field of 5 T, the critical current density in wire samples produced with application of the ECMAP method is enhanced approximately by a factor of 2 in comparison with the values obtained for the samples produced without the ECMAP treatment.     

Effect of Electrical Discharge Machining on strength and reliability of WC–30%Co composite

Tungsten carbide/Cobalt (WC–Co) composite is one of the important composite materials, which is used for manufacturing of cutting tools, dies and other special tools. It has very high hardness and excellent resistance to shock and wear. It is not possible to machine this material easily with conventional machining techniques. Due to the good electrical conductivity of WC–Co, it is usually machined by Electrical Discharge Machining (EDM). EDM process often results in the surface damage of bulk WC–Co, and the influence of the damage would affect the reliability. It is important to investigate the effect of electric discharge machining process on the properties of WC–Co cemented carbides before applying its engineering application. For these composites, maintenance of proper fracture strength is an important concern and is to be controlled. In this work, an attempt has been made to investigate the fracture strength and the reliability of EDMed WC–Co composite using the Weibull distribution analysis. The comparison of results between the machined composites and un-machined composites is carried out and presented in this study.